1. Field of the Invention
The present invention relates to an electronic endoscope system.
2. Description of the Related Art
Conventionally, electronic endoscope systems are widely used in medical diagnosis and treatment. The electronic endoscope system is constituted of an electronic endoscope having an insert section, a processor device and a light source device connected to the electronic endoscope, and the like.
The electronic endoscope is provided with the insert section to be introduced into a patient's body cavity and an operation section for operating the insert section. In a distal end portion of the insert section, there are formed a lighting window through which illumination light is applied in the patient's body, and an imaging window through which the inside of the body is imaged. In the electronic endoscope, an image sensor captures through the imaging window an image of an internal body portion to be examined under the illumination light applied through the lighting window. The processor device applies various types of processing to an image signal outputted from the image sensor, and produces an observation image used for the diagnosis. The observation image is displayed on a monitor connected to the processor device. The light source device has an adjustable-intensity white light source to supply the electronic endoscope with the illumination light. The illumination light is led to the distal end portion through a light guide routed through the electronic endoscope, and is applied from the lighting window to the internal body portion through a lighting optical system.
The insert section is required to have a small diameter, with the aim of reducing a physical burden on the patient. For this reason, the distal end portion contains the single image sensor in general, but there is known an electronic endoscope system that contains plural image sensors in the distal end portion to improve clarity of the image (refer to Japanese Patent Laid-Open Publication Nos. 2004-40480 and 2008-79823). For example, the Japanese Patent Laid-Open Publication No. 2004-40480 discloses a technique in which a half mirror splits an optical path in two directions, and two image sensors disposed in the two paths capture images with different exposure times. A composite image of the two images facilitates expanding a dynamic range. The Japanese Patent Laid-Open Publication No. 2008-79823 discloses a technique in which a dichroic prism splits incident light into green light (G light), red light (R light), and blue light (B light). An image of the G light is merged with an image of the R and B light to improve resolution.
In recent years, there is known a method in which the internal body portion is imaged with application of specific wavelength light (hereinafter called special light), to emphasize specific tissue such as a tumor and a capillary vessel in a superficial mucous membrane. For example, in narrow band imaging, blue light in a narrow wavelength band is applied to the internal body portion, and its reflected light is captured. In autofluorescence imaging, autofluorescence that is emitted from specific tissue inside the human body in response to application of excitation light is captured. In agent fluorescence imaging, a fluorescent labeling agent is administered to the patient in advance, and agent fluorescence that is emitted from the labeling agent in response to application of excitation light is captured. In infrared imaging, infrared light is applied to the internal body portion, and its reflected light is captured.
Among the light to be captured as described above, the autofluorescence, the agent fluorescence, or the infrared light produces a weak signal. To image such weak light, a specific image sensor is provided in U.S. Pat. No. 8,007,433 corresponding to Japanese Patent Laid-Open Publication No. 2007-50106. In the U.S. Pat. No. 8,007,433, a prism splits light incident from the internal body portion in two with respect to the wavelength of 500 nm. Then, two types of images, that is, an image of light with wavelengths of 500 nm or less and an image of light with wavelengths of more than 500 nm are captured separately, to obtain a high-quality special light image.
In capturing the weak fluorescence emitted from the internal body portion, even if the special light such as the excitation light emitted from the light source is filtered out and only the fluorescence is captured, a clear image suitable for the diagnosis is not necessarily obtained because the fluorescence itself is too weak. Thus, in the Japanese Patent Laid-Open Publication No. 2008-79823, for example, a signal obtained from one of two image sensors is amplified to produce an autofluorescence image. The amplification of the signal by image processing, however, has a disadvantage that a noise component is also amplified proportionally. Therefore, when the weak light such as the autofluorescence and the agent fluorescence is captured, it is desired that a signal of the light to be captured itself is amplified to obtain a clear image, in addition to cutting out the special light.